Nutrient absorption potential and nutrient resorption
We first defined a new index to assess the nutrient absorption potential of absorptive roots by integrating the key root traits (BI, ARL, and SRL) closely associated with nutrient absorption (Hodge 2004; Kong et al. 2014; Liese et al. 2017) and root-soil accumulation factor (Kou et al. 2017). First, we multiplied BI (tips cm-1) by ARL (cm) to obtain the total length of absorptive roots per unit length of the third-order root. We then divided the total length by SRL (m g-1) to obtain the estimated biomass allocation to absorptive roots per unit length of the third-order root. Lastly, we multiplied the estimated biomass of absorptive roots by a coefficient (i.e. root-soil accumulation factor) to represent the nutrient absorption potential of absorptive roots per unit length of third-order root (g cm-1). Accordingly, the absorption potential of the absorptive roots was calculated using Eqs. (1) for N (nitrogen absorption potential–NAP) and (2) for P (phosphorus absorption potential–PAP), respectively.
NAP (g cm-1) = BI × ARL/SRL× [NAR/Ns] × 10, (1)
PAP (g cm-1) = BI × ARL/SRL × [PAR/Ps] × 10, (2)
where NAR and PAR are the N and P concentration in absorptive roots, Ns and Ps are the inorganic N (NH4+-N plus NO3--N) and AP concentration in soils.
The nutrient resorption efficiency was estimated as the percent difference between nutrient concentrations in green and senesced foliage (Aerts et al. 1996; Vergutz et al. 2012), calculated as Eqs. (3) and (4)
NRE (%) = (NGr-NSe)/NGr × 100 (3)
PRE (%) = (PGr-PSe)/PGr× 100 (4)
where NRE and PRE are the N and P resorption efficiency, respectively. NGr, PGr, NSe, and PSe are N and P concentrations in green and senesced leaves, respectively.
Nutrient resorption proficiency is defined as the extent to which the nutrient concentration (mg g-1) is reduced in senesced leaves. Lower litter nutrient concentrations correspond to higher nutrient resorption proficiency and vice versa (Killingbeck 1996). NRP and PRP are resorption proficiency of N and P, respectively.